Applications of gene therapy for the treatment of cancer hold tremendous promise. The potential effectiveness of cancer gene therapies depends, in part, on the efficiency and specificity of therapeutic gene transfer. One strategy that may be effective despite the current limitations of somatic gene transfer is the treatment of local disease by in vivo gene transfer of HSVTK. The concept is to transfer the HSVTK gene into a subpopulation of malignant cells rendering them and adjacent non-transduced cells sensitive to the drug gangciclovir. We will focus on animal and clinical models of mesothelioma to study and evaluate this strategy. This project will develop and evaluate the methods of gene transfer that will be required for successful gene therapies for mesothelioma based on HSVTK bystander approach. Gene transfer by recombinant adenoviruses and recombinant adeno associated viruses (AAV) will be evaluated. A variety of recombinant adenoviruses will be constructed that vary with respect to structure and function of the viral gene loci that have been retained in the recombinant. Our preliminary studies indicate that subtle differences in recombinant adenoviral structure can translate into significant changes in biology with respect ot replicative capacity, immunogenicity, and transgene persistence. The use of recombinant AAV will also be evaluated. This family of recombinant viruses has the advantage of conferring longer and more stable transgene expression with less immunogenicity. Both vector systems will be evaluated in several animal models of mesothelioma with respect to biological issues of 1) efficiency of gene transfer within the tumor, 2) specificity of gene transfer, 3) stability of gene trans, 4) immune responses to the target cells, and 4) associated pathology. This project will collaborate closely with Project II by providing HSVTK adenovirus and AAV technology as well, in addition to delineating the immune components that contribute to the bystander effect.